As is well known to those skilled in the art, it is possible to separate mixtures of liquids by various techniques including adsorption or distillation. These conventional processes, particularly distillation, are however characterized by high capital cost. In the case of distillation for example, the process requires expensive distillation towers, heaters, heat exchangers (reboilers, condensers, etc.), together with a substantial amount of auxiliary equipment typified by pumps, collection vessels, vacuum generating equipment, etc.
Such operations are characterized by high operating costs principally costs of heating and cooling--plus pumping, etc.
Furthermore the properties of the materials being separated, as is evidenced by the distillation curves, may be such that a large number of plates may be required, etc. When the charge components form an azeotrope, additional problems may be present which for example, could require that separation be effected in a series of steps (e.g. as in two towers) or by addition of extraneous materials to the system.
There are also comparable problems which are encountered in adsorption systems.
It has been found to be possible to utilize membrane systems to separate mixtures of miscible liquids by pervaporation. In this process, the charge liquid is brought into contact with a membrane film; and one component of the charge liquid preferentially permeates the membrane. The permeate is then removed as a vapor from the downstream side of the film--typically by sweeping with a carrier gas or by reducing the pressure below the vapor pressure of the permeating species.
Illustrative membranes which have been employed in prior art techniques include those set forth in the following table:
TABLE ______________________________________ Separating Layer References ______________________________________ Polyvinyl alcohol Kuraray Co. Japanese containing glycerine Patent 81/193495 (1981) JP 58/g5522A2 (1983) Nafion brand of Cabasso and Liu perfluorosulfonic acid J. Memb. Sci. 24, 101 (1985) Polytetrafluorethylene Neal, Aptel, and film grafted with N--vinyl Clement Desalination pyrolidone 53, 297, (1985) Sulfonated polyethylene Cabasso, Korngold & Liu, J. Pol. Sci.: Letters, 23, 57 (1985) Fluorinated Polyether U.S. Pat. No. 4,526,948 or Carboxylic Acid to DuPont as assignee Fluorides of Resnickto Selemion AMV Wentzlaff blend of Asahi Glass Boddeker & Hattanbach cross-linked styrene J. Memb. Sci. 22,333 butadiene (with quaternary (1985) ammonium residues on a polyvinyl chloride backing Cellulose triacetate Wentzlaff, Boddeker & Hattanback, J. Memb. Sci. 22, 333 (1985) Polyacrylontrile or Neel, Aptel & Polytetrafluoroethylene Clement Desalination grafted with N--vinyl 53, 297 (1985) pyrrolidone Cellulose Acetate Proc. of Int. Mem. and others Conf. Sept., 1986 Ottawa, p 229 Crosslinked Eur. Patent 0 096 Polyvinyl Alcohol 339 to GFT as assignee of Bruschke Poly(maleimide- Yoshikawa et al acrylonitrile) J. Pol. Sci. 22, 2159 (1984) Dextrine - Chem. Econ. Eng isophoronediisocyanate Rev., 17, 34 (1985) ______________________________________
The cost effectiveness of a membrane is determined by the selectivity and productivity. Of the membranes commercially available, an illustrative membrane of high performance is that disclosed in European patent 0 096 339 A2 of GFT as assignee of Bruschke--published 21 Dec. 1983.
European Patent 0 096 339 A2 to GFT as assignee of Bruschke discloses, as cross-linking agents, diacids (typified by maleic acid or fumaric acid); dihalogen compounds (typified by dichloroacetone or 1,3-dichloroisopropanol); aldehydes, including dialdehydes, typified by formaldehyde. These membranes are said to be particularly effective for dehydration of aqueous solutions of ethanol or isopropanol.
This reference discloses separation of water from alcohols, ethers, ketones, aldehydes, or acids by use of composite membranes. Specifically the composite includes (i) a backing typically about 120 microns in thickness, on which is positioned (ii) a microporous support layer of a polysulfone or a polyacrylonitrile of about 50 microns thickness, on which is positioned (iii) a separating layer of crosslinked polyvinyl alcohol about 2 microns in thickness.
Polyvinyl alcohol may be cross-linked by use of difunctional agents which react with the hydroxyl group of the polyvinyl alcohol. Typical cross-linking agent may include dialdehydes (which yield acetal linkages), diacids or diacid halides (which yield ester linkages), dihalogen compounds or epichlorhydrin (which yield ether linkages) olefinic aldehydes (which yield ether/acetal linkages), boric acid (which yields boric ester linkages), sulfonamidoaldehydes, etc.
See also J. G. Prichard, Polyvinyl Alcohol, Basic Properties and Uses, Gordon and Breach Science Publishers, New York (1970) or
C. A. Finch, Polyvinyl Alcohol, Properties and Applications, John Wiley and Sons, New York (1973).
It is an object of this invention to provide a separation process. Other objects will be apparent to those skilled in the art.